The ionic conduction mechanism of the compound 4CaO . P2O5 has been investigated by means of the electrolysis, the dependence of electric conductivity on oxygen partial pressure and the electromotive force of the Galvanic cells using CaO-Al2O3 compounds or molten Ag-Ca alloys as electrodes. It is concluded that 4CaO . P2O5 is a calcium ion conductor under oxygen partial pressure from 1 x 10(5) to 1 x 10(-5) Pa at temperatures between 1373 and 1873 K and under Ca vapor equibrated with Ag-Ca molten alloy.